Artificial lift system

ABSTRACT

A method and system for artificially lifting enriched solvents in solution mining wells are disclosed. A liquid immiscible with and lighter in gravity than the solvent is injected in the well annulus while the solvent enriched with minerals is produced through the well tubing. The system can selectively be placed in continuous or intermittent operation.

[ 51 Apr. 4, B972 ited Sines Blumenburg [54] ARTIFICIAL LIF T SYSTEM299/5 ...417/55 McLeod E 755 246 999 1.1.1 171. l .l 597 838 905 400 23[73] Assignee: Esso Production Research Company, Primary Exammer EmestRpurser Houston, Tex.

AttorneyJames A. Reilly, John B. Davidson, Lewis H. Eatherton, James E.Gilchrist, Robert L. Graham and James E. Reed [22] Filed: Sept. 29, 1969{57] ABSTRACT A method and system for artificially lifting enrichedsolvents App]. No.:

in solution mining wells are disclosed. A liquid immiscible with andlighter in gravity than the solvent is injected in the well annuluswhile the solvent enriched with minerals is 2 W9 mu u m4 and /1 we 6 6 1m f. a e s 0 d l .w F 1 8 5 produced through the well tubing. The systemcan selectively be placed in continuous or intermittent operation.

9 Claims, 3 Drawing Figures References Cited UNITED STATES PATENTS461,431 Frasch PATENTEDAPR 4 I972 3,653,717

sum 1 or 3 I N VEN I'ORS Elvis Rich Edgar L. VonRoseqQerg BY%M//L//WATTORNEY PATNTEDAFR 4 I972 SHEET 2 OF 3 1 I l l l l l l I l I I I .l

INVENTORS Elvis Rich BY E djr L. VonRos/en7berg AT TORNE Y PATENTEDAPR 4I972 3,653.71?

sum 3 (1F 3 r I l I l I l l L I N VEN 'OR 5 Elvis Rich E dgqr L.vonRosenllefg ATTORNEY ARTIFICIAL LIFT SYSTEM BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates generally toartificial lifting of liquid from a subsurface elevation and moreparticularly to artificial lift means for solution mining.

2. Description of the Prior Art Solution mining is a mineral recoveryprocess which utilizes a leaching solvent for dissolving the mineral insitu. In one form, it involves the injection of the leaching solventinto the mineral-bearing formation through selected input wells and therecovery of the enriched solvent through selected producing wells. Themineral can be present in the matrix of a host rock or asa massivedeposit. In either situation, communication between the input andproducing wells can be induced or improved by fracturing techniquesdeveloped in the petroleum industry. Owing largely to advancedmetallurgical and formation treating technology, solution mining, oflate, has been applied in the in situ recovery of uranium and has beenproposed for the recovery of a host of other minerals such as copper,phosphate, and manganese. As the technology continues to advance and asthe demand for minerals increases accompanied by the inexorabledepletion of present reserves, it is reasonable to expect the continuedexpansion of solution mining techniques.

While solution mining offers significant advantages over excavationmining-particularly in the treatment of low-grade oresit is beset by theserious and continuing problem of corrosion. Solvents having thecapabilities of selectively dissolving the desired minerals generallyare highly corrosive. For example, uranium is leached in situ by aqueoussolutions of nitric or sulfuric acid, the corrosivities of which arewell known. Even in the solution mining of salt, the corrosivity of theproduced brine requires the use of special corrosive-resistantequipment.

In the recovery phase of the operation, the enriched solvent must belifted from the subterranean formation. A prime consideration inselecting the type of artificial lift system is the systems ability tohandle the highly corrosive solvents. Conventional artificial liftfacilities include submersible pumps and air lifts. Both of thesefacilities require the exposure of substantial amounts of equipment tothe corrosive liquids and therefore experience relatively shortoperating lives. The repair and replacement of subsurface pumps is aparticularly expensive operation requiring the withdrawal of the entireassembly. While the air-lift system obviates some of the operationaldisadvantages of the subsurface pump, particularly in regards to thereplacement of parts, it presents other problems. The commingled air andsolvent resulting from the air lift, in fact, increases the rate ofcorrosion so that while replacement of parts is facilitated, thefrequency of replacement is increased.

SUMMARY OF THE INVENTION The present invention contemplates lifting thesolvent by injecting a power liquid down the well annulus and producingthe solvent through the well tubing. The subsurface equipment includes apacker and check valve assembly for permitting flow into the casingwhile preventing backflow during the lifting phase of the cycle.

Under certain conditions, the formation may have sufficient pressure tocause the static fluid level to stand high in the well. Under theseconditions flow can be induced by reducing the pressure gradient in thefluid column such that the back pressure on the formation is less thanthe formation pressure. The present invention contemplates thecontinuous commingling of a light liquid and solvent in the producingstring. The rate of production attainable by this continuous lift systemdepends upon the pressure draw-down imparted on the formation, which inturn depends upon the difference in liquid and solvent densities and therelative volumes commingled.

As the formation pressure is dissipated by the withdrawal of formationfluids, the differential pressure gradually declines to a point that amore positive lift may be required in order to maintain the desiredproduction rate. The surface facilities may then be modified to providean intermittent lift which operates on a displacement principle. Inintennittent lift operation, the solvent is permitted to rise in theannulus while maintaining a column of light liquid above the solventfluid level. When the solvent fluid level reaches a predetermined upperlevel in the annulus, a power liquid which can be the same compositionas the light liquid is injected in the casing forcing the solvent downthe annulus and up the tubing. The solvent fluid level is displaceddownwardly within the annulus to a predetermined lower elevationwhereupon injection is discontinued placing the system in a conditionfor repeating the lift cycle. Thus, the lift cycle comprises l) asolvent entry phase where solvent from the formation enters the wellannulus rising to a predetermined elevation therein and (2) adisplacement phase where a power liquid displaces solvent in the annulusforcing it up the well tubing.

A particular advantageous feature of the lifting system contemplated bythe present invention is the use of few moving parts exposed to thecorrosive solvent. The intermittent lift system has the added advantageof permitting the accurate control of injection of the power liquid.Moreover the intermittent operation does not entail commingling of thepower liquid and solvent and therefore requires 'no surface separation.

The injection of the power liquid when in intermittent operation can bemade operatively responsive to the solvent fluid level in the annulus sothat the operation thereof is a function of the producing rate of thewell. The amount of power liquid injected in each displacement phase ofthe cycle is regulated by accurate volumetric control.

By way of summary then the present invention provides a novel method andsystem for lifting corrosive liquids, the system being adapted forcontinuous or intermittent operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic viewillustrating the artificial lift system according to the presentinvention under continuous operation; and

FIGS. 2 and 3 are diagrammatic views illustrating the artificiaI liftsystem under intermittent operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1 a well10 is shown completed for draining a subterranean formation 11. Theformation 11 is mineral-bearing having characteristics such that it isamenable to the solution mining process. The mineral deposits may bepresent in the matrix of a host rock or may be present as a massivecontinuous deposit. The mineral can be uranium, copper, phosphate, salt,or other mineral which can be extracted by in situ leaching according topresently known techniques. The formation 11 can have natural matrixpermeability or can be fractured for improving the permeability. Ineither event there is sufficient permeability for conducting theleaching solvent from an adjacent input well (not shown) to theproducing well 10. The usual solution mining operation involvesinjecting a mineral dissolving or leaching solvent via the input wellinto the mineral-bearing formation 11 and withdrawing the solventenriched with the mineral at the producing well 10.

The well 10 is drilled and completed by techniques well known in thepetroleum industry. In completing the well 10 as depicted, a borehole 12is drilled to a depth sufficient to penetrate the formation 11. Casing13 is run to the top of the formation 11 and cemented in place. A packer14 carried at the lower end of the tubing string 15 is set at the baseof the casing 13.

Owing to the high density of the enriched solvent, the formationpressure may not be sufficient to cause the well to flow at economicrates. In this event, an artificial lift system is required. Because ofthe highly corrosive nature of the solvent, it is desirable that thelift system employed have few moving subsurface parts exposed to thesolvent.

The present invention contemplates lifting the enriched solvent throughthe tubing 15 by injecting a low-density liquid into the annulus 17defined by the casing 13 and tubing 15. The surface facilities forinjecting the light liquid are designed to permit either continuous orintermittent operation. The subsurface equipment can accommodate eithertype of operation without modification.

More specifically, the tubing string 15 comprises, from bottom up, alower landing nipple 18 secured to the packer 14, a perforated nipple19, an upper landing nipple 20 disposed immediately above the perforatednipple 19, and a continuous conduit 21 to the surface. The landingnipples l8 and 20 are adapted to receive wireline retrievable standingvalves 22 and 23, respectively.

When the well is placed on production the formation pressure forcessolvent into the wellbore 12 and up the tubing string 15. If theformation pressure is greater than the back pressure imposed on theformation face by a fluidcolumn in the tubing string 15, the well flowsby natural process. The pressure in formation 11 can be due to thehydrostatic pressure imposed by the weight of water indigenous to theformation or overburden pressure resulting from the weight of upperformations. Or the formation 11 can be pressurized by injection offluids in the input wells.

If the formation pressure is sufficiently high to provide a static fluidlevel close to the surface, flow can be induced by reducing the pressuregradient in the tubing string 15. The present invention, in one aspect,contemplates a continuous lift process involving the commingling of alight liquid with the produced solvent in the tubing string 15.Preferably the light liquid is immiscible with the solvent so as tofacilitate the surface separation of the liquid and solvent.Accordingly, a light lift liquid is continuously injected down theannulus 17 at such a rate to cause liquid to enter perforated nipple 19and intermix with the solvent stream in string 15. The rate of injectionis controlled to provide the proper mixture of lift liquid and solventto give the desired pressure gradient. This reduces the formation backpressure, imparting a pressure draw-down on the formation 11. The ratioof lift liquid to solvent to effect a given pressure gradient in thetubing column of course will depend upon their respective densities. Ifthe densities of the solvent and lift liquid are such to separatelyprovide a pressure gradient of 0.54 psi per foot (specific gravity of1.25) and 0.35 psi per foot (specific gravity of 0.80), respectively, amixture of two parts of the former to one part of the latter provides apressure gradient of 0.48 psi per foot. Thus if the formation 11 has astatic fluid column of 2,000 feet, a pressure draw-down of 120 psi canbe imparted on the formation by the continuous injection of a lightliquid at a rate equal to one-third the total fluid withdrawal rate.

The continuous lift process described above is applicable in wellscompleted in formations having relatively high pressures. Its primarydisadvantage is the limitation on the pressure draw-down on theformation.

As the formation pressure declines, the flowing gradient must bedecreased correspondingly to maintain the desired level of production.Lower pressure gradients are obtained by increasing the ratio of liftliquid to solvent. At a point where an excessive amount of liquid isrequired to maintain the desired pressure draw-down, it may be moreeconomical to switch to the intermittent operation. While theintermittent operation reduces the flowing time of the well, it permitsa greater pressure draw-down on the formation which should more thanoffset the losses resulting from the nonproducing time intervals of thelifting cycle.

The intermittent lift system according to the present invention operateson the principle of displacement. Under dynamie lift conditions, solventfrom the formation 1 1 is permitted to rise in the annulus 17 while acolumn of power liquid is maintained to the surface above the solventfluid level. The selected power liquid is immiscible with and lighter ingravity than the solvent so that the former floats on the latter, thecontact being at the interface 25 shown in FIG. 2. At a predeterminedupper elevation of the interface 25, the power liquid is injected intothe annulus 17. This displaces the interface 25 downwardly within theannulus 17 forcing solvent through the perforated nipple 19 and up thetubing string 15 (see H6. 3). At a predetermined lower elevation of theinterface 25, liquid injection is discontinued and the system is placedin a condition for repeating the lift cycle. Thus the back pressure onthe formation 11 with the interface 25 in the lower position, isprincipally the product of the pressure gradient of the power liquid andthe depth of the well. In the well of the above example wherein thesolvent and liquid have pressure gradients of 0.54 and 0.35 psi perfoot, the pressure draw-down on the formation can be increased by about370 psi. If the formation pressure balances a static fluid column 1,500feet in the tubing string, a pressure draw-down of psi is attainable bythe intermittent lift system.

The subsurface equipment in the well 10 is identical for either thecontinuous or intermittent operation. It should be noted that the onlymoving parts are the two standing valves 22 and 23 which are easilyretrieved by conventional wireline equipment. If the flow area of theannulus 17 is substantially greater than the flow area of the tubing 15,valve 23 is not required. However its presence provides for a moreefficient displacement operation since it prevents backflow from thetubing 15 during the solvent entry phase of the lift cycle.

In either the continuous or intermittent operation, the pressuredraw-down attainable is a function of the relative lift or power liquidand the solvent. The produced solvents laden with the extracted mineralgenerally will have a specific gravity in the order of 1.25, but couldhave a specific gravity as low as 1.1. The wider the separation ofgravities, the greater the draw-down on the formation. Accordingly it ispreferred that the differential of specific gravities be at least 0.2.

In solution mining operations where the solvent is an aqueous solutionof a mineral-dissolving material such as that used in the solutionmining of salt or uranium, a liquid satisfying the requirements ofimmiscibility and light gravity is found in the light petroleumfractions such as gasoline, kerosene, or diesel fuel. These hydrocarbonliquids have the property of being stable at the normal operatingtemperatures and pressures so that they can be recycled therebyproviding a closed system.

The surface facilities are designed to permit either type of operation.Considering first the continuous lift operation (FIG. 1), the componentsinclude a separator 26, a reservoir 27, and a motor-driven pump 28, andthe necessary piping for interconnecting the parts as depicted. Throughsuitable connections, the pump 28 receives the lift liquid from thereservoir 27 and delivers it to the annulus 17. The rate of injectioncan be calculated or can be regulated to achieve the desired solventproducing rate. The produced mixture of solvent and lift liquid isdirected to the separator 26 where the lighter liquid is returned by anoverhead line to the reservoir 27 while the heavier solvent isdischarged to storage. An interface level controller 30 is operativelyconnected to the separator discharge valve 31 and serves to maintain theinterface within the confines of separator 26.

The surface equipment for the intermittent operation includes thereservoir 27 and pump 28 plus a motor valve 29 and liquid levelcontrollers 32 and 33 (see FIGS. 2 and 3). The level controllers 32 and33, respectively, provide high-level and low-level control points withinthe reservoir 27. The pump 28 through suitable controls is operativelyresponsive to actuation of the high-level controller 32. The motor valve29 is a three-way, two-position, directional valve having a firstposition which provides fluid communication between the pump 28 andannulus 17 and a second position which provides fluid communicationbetween the reservoir 27 and the annulus 17.

The valve 29 is normally in the first position and is energized to thesecond position by actuation of the low-level controller 33.

At the beginning of the lift cycle, occasioned by the actuation of thelow-level controller 33, the energized motor valve 29, in the secondposition, permits the flow of power liquid from the annulus 17 to thereservoir via suitable conduits. The solvent entering the annulus 17through the lower standing valve 18 and perforated nipple 19 displaces alike amount of power liquid into the reservoir 27. The solvent fluidlevel 26 rises in the annulus 17 until sufiicient power liquid has beendisplaced in the reservoir 27 to actuate the high-level controller 32.This deenergizes the motor valve 29 and activates the pump 28, asuitable time delay being provided if necessary. Power liquid is pumpeddown the annulus l7 depressing the interface 25 therein until apredetermined liquid slug has been injected as determined by thereservoir volume between the upper and lower level controllers 32 and33. The solvent, being restricted from backtlowing into the formation 11by the lower standing valve 18, is displaced up the tubing string anddischarged at the surface to storage, standing valve 23 preventingbackfiow from the tubing 15. The volume of each slug can vary within awide range. In a well having 7-inch casing and 2-inch tubing, a slug of150 gallons provides an annular displacement of 100 feet. Since thepower liquid is not commingled with the solvent, the separator 26 can bebypassed as illustrated. It should be observed that the dis placementmedium (power liquid) being essentially an incompressible liquid permitsthe actuation of the displacement phase of the lift cycle in response tothe producing ability of the formation 11. In other words the injectionof the power liquid does not commence until a predetermined volume ofsolvent has entered the annulus 17. Moreover, by simply adjusting thespace between the high-level and low-level controllers, 32 and 33, thevolume of each slug of power liquid injected can be varied to meet avariety of producing conditions. The individual components of thesurface facilities, reservoir 27, pump 28, valve 29, and associatedcontrols are conventional and except for the combination claimed form nopart of the present invention.

In describing the operation of the intermittent lift system, let it beassumed that the upper and lower pumping levels are those shown in FIGS.2 and 3, respectively. As indicated above, these operating levels of theinterface 25 are determined by the relative positions of the highandlow-level controllers within reservoir 27 and that by simple adjustmentthe operating levels can be changed. At the beginning of the solvententry phase of the lift cycle, the tubing 15 is full of the solventbeing retained by the standing valve 23, and the annulus 17 is filledprincipally with the lighter power liquid. Thus the static head imposedon the formation 11 is substantially less than the formation pressure.Accordingly solvent enters the wellbore 12, passes through the standingvalve 22, through the perforated nipple 22 and up the annulus 17. Theinterface 25 rises displacing the power liquid ahead of it forcing itinto the reservoir 27. Interface 25 rises a predetermined elevation inthe annulus 17 determined by the location of the high-level controller32. Actuation of the high-level controller places the system incondition for performing the displacement phase of the cycle. A slug ofpower liquid is injected in the annulus 17 depressing the interface 25to the low-level elevation forcing the solvent up the tubing 15 andeventually to the storage facilities. The injection rate is considerablyfaster than the fluid entry rate so that the solvent entry phase of thelifting cycle is considerably longer in duration than the displacementphase.

Summarizing, the present invention provides for an artificial liftmethod and system particularly adapted to the solution mining operationsand is characterized as having a minimum of moving parts disposed to thecorrosive solution mining solvents. The lift system can be used inintermittent or continuous operation. In intermittent operation, thesystem offers the added feature of displacing the solvent in the annuluswithout the necessity of commingling the displacing medium and theproduced solvent, and provides means for actuating the system responsiveto the producing ability of the well.

We claim:

1. A method for lifting a corrosive aqueous solution in a cased wellcompleted for draining a mineral-bearing formation, and having a tubingstring disposed therein, said formation having sufficient pressure toprovide a static fluid column which substantially fills said tubingstring, said method comprising continuously introducing a hydrocarbonliquid into the lower end of said tubing string, said hydrocarbon liquidbeing immiscible with said aqueous solution and having a densitysubstantially less than the density of said aqueous solution, the volumeof said hydrocarbon liquid introduced into said tubing string beingsufficient to substantially reduce the pressure gradient of said fluidcolumn therein to permit the aqueous solution to be produced.

2. The method as recited in claim 1 wherein the aqueous solution has aspecific gravity greater than about LI and the liquid hydrocarbon has aspecific gravity less than about 0.9 at operating conditions. 7

3. The method as recited in l and further comprising the steps ofseparating the produced aqueous solution and hydrocarbon liquid, andreintroducing the hydrocarbon liquid into the lower end of said'tubing.

4. A method of lifting a mineral solvent in a cased well completed fordraining a mineral-bearing formation and having a tubing string disposedtherein, the lower end of said tubing string being in fluidcommunication with the casing-tubing annulus, said method comprising therepetitive steps of permitting the solvent to enter the annulus underformation pressure while maintaining said annulus above the solventfluid level therein full of a power liquid immiscible with and lighterin gravity than said solvent; and thereafter injecting a power liquidinto the annulus while preventing backflow of the solvent into theformation to displace solvent received in said annulus through saidlower end and into said tubing string.

5. The method of claim 4 wherein the power liquid is a hydrocarbonliquid having a specific gravity less than about 0.9 and adapted for usein solution mining wells using an aqueous solution of a mineral solventhaving a specific gravity greater than about 1.1.

6. The method of claim 5 wherein the volume of power liquid injectedinto said annulus is volumetrically controlled to prevent power liquidfrom entering said tubing string.

7. A system for lifting a mineral solvent in a cased well completed fordraining a mineral-bearing formation, said system comprising a tubingstring disposed in said well and having an inlet at its lower end; valvemeans connected to an upper portion of the annulus defined by the wellcasing and said tubing string for controlling fluid flow into and out ofsaid upper portion of said annulus; pump means for injecting a powerliquid through said valve means and into said annulus, said power liquidbeing immiscible with and lighter in gravity than said solvent; controlmeans for operating said pump means and said valve means in a repeatingpump cycle which comprises a solvent entry phase wherein pressure isrelieved from said annulus through said valve means permitting solventto flow from said formation into a lower portion of said annulus, and alift phase wherein said pump means is operated to inject power liquidinto said annulus to displace solvent from said lower portion of saidannulus into said tubing string.

8. The system of claim 7 wherein said pump means includes a pump, apower liquid reservoir connected to said pump, piping connecting saidpump and said reservoir to said valve means, said valve means includes amotor valve having a first position providing fluid communicationbetween said pump and said annulus and a second position providing fluidcommunication between said annulus and said reservoir, and said controlmeans includes first control means for operating said pump and forplacing said motor valve in said first position attendant to saidsolvent reaching a predetermined upper elevation in said annulus andsecond control means for deactivating said pump and for placing saidmotor valve in said second position attendant to said solvent beingdisplaced to a predetermined lower elevation in said annulus.

9. The system as recited in claim 8 wherein said first and secondcontrol means include high-level and low-level controllers disposed insaid reservoir.

2. The method as recited in claim 1 wherein the aqueous solution has aspecific gravity greater than about 1.1 and the liquid hydrocarbon has aspecific gravity less than about 0.9 at operating conditions.
 3. Themethod as recited in 1 and further comprising the steps of separatingthe produced aqueous solution and hydrocarbon liquid, and reintroducingthe hydrocarbon liquid into the lower end of said tubing.
 4. A method oflifting a mineral solvent in a cased well completed for draining amineral-bearing formation and having a tubing string disposed therein,the lower end of said tubing string being in fluid communication withthe casing-tubing annulus, said method comprising the repetitive stepsof permitting the solvent to enter the annulus under formation pressurewhile maintaining said annulus above the solvent fluid level thereinfull of a power liquid immiscible with and lighter in gravity than saidsolvent; and thereafter injecting a power liquid into the annulus whilepreventing backflow of the solvent into the formatioN to displacesolvent received in said annulus through said lower end and into saidtubing string.
 5. The method of claim 4 wherein the power liquid is ahydrocarbon liquid having a specific gravity less than about 0.9 andadapted for use in solution mining wells using an aqueous solution of amineral solvent having a specific gravity greater than about 1.1.
 6. Themethod of claim 5 wherein the volume of power liquid injected into saidannulus is volumetrically controlled to prevent power liquid fromentering said tubing string.
 7. A system for lifting a mineral solventin a cased well completed for draining a mineral-bearing formation, saidsystem comprising a tubing string disposed in said well and having aninlet at its lower end; valve means connected to an upper portion of theannulus defined by the well casing and said tubing string forcontrolling fluid flow into and out of said upper portion of saidannulus; pump means for injecting a power liquid through said valvemeans and into said annulus, said power liquid being immiscible with andlighter in gravity than said solvent; control means for operating saidpump means and said valve means in a repeating pump cycle whichcomprises a solvent entry phase wherein pressure is relieved from saidannulus through said valve means permitting solvent to flow from saidformation into a lower portion of said annulus, and a lift phase whereinsaid pump means is operated to inject power liquid into said annulus todisplace solvent from said lower portion of said annulus into saidtubing string.
 8. The system of claim 7 wherein said pump means includesa pump, a power liquid reservoir connected to said pump, pipingconnecting said pump and said reservoir to said valve means, said valvemeans includes a motor valve having a first position providing fluidcommunication between said pump and said annulus and a second positionproviding fluid communication between said annulus and said reservoir,and said control means includes first control means for operating saidpump and for placing said motor valve in said first position attendantto said solvent reaching a predetermined upper elevation in said annulusand second control means for deactivating said pump and for placing saidmotor valve in said second position attendant to said solvent beingdisplaced to a predetermined lower elevation in said annulus.
 9. Thesystem as recited in claim 8 wherein said first and second control meansinclude high-level and low-level controllers disposed in said reservoir.